Performance Of Electromagnetic Acoustic Transducers Research Articles

Lift-off performance of electromagnetic acoustic transducers (EMATs) for surface acoustic wave generation

Electromagnetic acoustic transducers (EMATs) have a behaviour that depends on the distance between the transducer and the surface of the material under test, namely the lift-off. Rayleigh waves generated by EMATs suffer from waveform distortion as the lift-off is increased. This paper describes this distortion numerically and experimentally, focusing on the spatial distribution of the induced currents and hence the ultrasound pulses. This is verified experimentally using an EMAT consisting of a magnet and a single 1.5 mm wide linear coil, showing a decrease in peak frequency for the wideband Rayleigh wave of the order of 100 kHz/mm with lift-off. The behaviour when using EMATs in an array configuration (equivalent to a meander-coil, where the current through neighbouring coils is in opposite directions) is then described. Coil spacing is shown to affect the lift-off behaviour, with the spatial model predicting well the frequency behaviour. The lift-off behaviours of physically spaced coils and those which are pulsed with time delays to give an effective separation are shown to be equivalent for all but the smallest separations; unless very small separations are required to give a high frequency signal, the effective spatial current distribution spread for a single coil counteracts the benefits of reducing the dipole effect. Good performance with lift-off variation is hard to achieve at high frequency, and lift-off must be considered if a particular frequency of operation is required.

Orthogonal Optimal Design Method for Spiral Coil EMAT Considering Lift-off Effect: A Case Study

This paper provided a method to improve the conversion efficiency and lessen the lift-off effect of spiral coil electromagnetic acoustic transducer (EMAT) for some specific situations, for example, irregular sample surface, online inspection and varying coating. Moreover, a new evaluation method for the performance of EMAT is proposed. To study the effect of the structural parameters on the performance of EMAT, the orthogonal test method with five factors and four levels is employed. Consequently, the degree of five factors on the lift-off performance and the theoretical maximum value of amplitude are obtained. The results show almost all factors have an obvious impact on the theoretical maximum value of amplitude, while only the magnet-to-coil distance can affect the lift-off performance effectively. The experiment results well agree with the numerical results, in which the improvement of the lift-off performance and the theoretical maximum value of amplitude is 31.94% and 78.79% respectively. In addition, the conversion efficiency of EMAT is increased by 125% when the lift-off is 1 mm. These imply the performance of spiral coil electromagnetic acoustic transducer is improved dramatically and the detectable lift-off is increased as well.

Development of EMA transducer for inspection of pipelines

Electromagnetic acoustic transducers (EMATs) have potential for In-line inspection (ILI) of pipelines compare to other Nondestructive evaluation (NDE) methods, since EMAT doesn’t need to contact and/or coupling material and can detect defects in outside of pipelines. However, improvement of transduction efficiency of EMATs is needed to secure integrity of the pipelines. To improve performance of EMAT, optimization of major parameters, such as strength of permanent magnet, shape of coil, configuration of static and dynamic magnetic fields is required. Thus, in this paper, investigation of transduction efficiency with variation of major parameters are performed by comparison of generated ultrasonic signals. Based on the investigation results, EMATs for ILI of pipelines are designed and fabricated with optimized Periodic solenoid array (PSA) types of meander-line sensor, static magnetic field, and then, performance of the developed EMATs are evaluated by detecting defects in the gas pipelines mounted in the one-bed testing system.

The Study on EMAT Applied in Nondestructive Testing of Elevated Temperature Material

Electromagnetic acoustic transducers (EMATs) has overcome many disadvantages of the traditional piezoelectric ultrasonic sensors and can be applied in elevated temperatures and on-line inspection. In an EMAT testing system, the tested material and EMAT itself make up a whole system. So the performance of EMAT is closely depended on some physical parameters of tested material. On the other hand, characteristic parameters of metallic materials are closely related to temperature, such as density, modulus of elasticity, electrical conductivity and magnetic permeability. As the temperature increases, the density of the metal material is reduced, the elastic modulus is reduced, the electrical conductivity is decreased, the magnetic permeability has a consistent increasing trend until the Curie point. Thus the EMAT detection performance affected by the change of temperature should be considered. This paper studied the effects of temperature on EMAT testing. Three materials of 20 # steel, Q235 and 16MnR were investigated. Testing temperature varies from 26 °C to 500 °C. The results show that under elevated temperature condition, EMAT echo signals still have a good waveform and stability. Meanwhile, signals are attenuated less than 2dB. It proved that the EMAT technique has a good steady performance in elevated temperature.

Excitation and detection of shear horizontal waves with electromagnetic acoustic transducers for nondestructive testing of plates

The fundamental shear horizontal(SH0) wave has several unique features that are attractive for long-range nondestructive testing(NDT). By a careful design of the geometric configuration, electromagnetic acoustic transducers(EMATs) have the capability to generate a wide range of guided wave modes, such as Lamb waves and shear-horizontal(SH) waves in plates. However, the performance of EMATs is influenced by their parameters. To evaluate the performance of periodic permanent magnet(PPM) EMATs, a distributed-line-source model is developed to calculate the angular acoustic field cross-section in the far-field. Numerical analysis is conducted to investigate the performance of such EMATs with different geometric parameters, such as period and number of magnet arrays, and inner and outer coil widths. Such parameters have a great influence on the directivity of the generated SH0 waves that arises mainly in the amplitude and width of both main and side lobes. According to the numerical analysis, these parameters are optimized to obtain better directivity. Optimized PPM EMATs are designed and used for NDT of strip plates. Experimental results show that the lateral boundary of the strip plate has no perceivable influence on SH0-wave propagation, thus validating their used in NDT. The proposed model predicts the radiation pattern of PPM EMATs, and can be used for their parameter optimization.

The impact of magnetostriction on the transduction of normal bias field EMATs

A very typical and important application of Electromagnetic Acoustic Transducers (EMATs) is the inspection of ferritic steels with normal bias field transducers. In this case, a controversy has arisen in the literature, as some older studies have indicated the Lorentz force as the main transduction mechanism, while more recent research has claimed that magnetostriction can be two order or magnitudes larger than the Lorentz effect. This is not merely an academic issue, as depending on which physical phenomena dominates, the performance of EMATs on different steel grades might significantly vary and the design of the transducer could be optimized accordingly.This paper analyzes in depth two main assumptions made in the more recent studies, highlighting some inconsistencies. A previously experimentally validated Finite Element model, is used to test the controversial assumptions. It is demonstrated that the mechanical boundary conditions were not modelled correctly leading to a gross overestimation of the role of magnetostriction. The main conclusion is that the magnetostriction force is typically not order of magnitudes larger than the Lorentz force; actually the Lorentz force is the larger transduction effect in non-oxidized ferromagnetic steels, and magnetostriction is only a fraction of it.

A Coupled Electromagnetic and Mechanical Analysis of Electromagnetic Acoustic Transducers

In this paper a comprehensive finite element analysis of Electromagnetic Acoustic Transducer (EMAT) is presented. The analysis has been performed using complete and incomplete expressions for excitation current and results have been compared with published data to assess the accuracy of the complete expression approach. Furthermore, the effects of varying the EMAT parameters, such as lift-off and coil dimensions, also the excitation frequency and static magnetic flux density, were investigated to show the effects they have on the performance of EMAT. Additionally, a finite element analysis was conducted on the coupled physics between electromagnetic and mechanical displacement of EMAT.

Electromagnetic acoustic transducers as wideband velocity sensors

The performance of electromagnetic acoustic transducers as wideband velocity sensors has been investigated, using a pulsed laser as a standard acoustic source.